There is a continuing need for a simple, reliable, low cost yet high performance fuel injection system which can effectively and predictably control both fuel injection timing and metering. However, the design of such a fuel injection system necessarily involves acceptance of some characteristics which are less than optimal since the basic goals of low cost, high performance and reliability are often in direct conflict. For example, distributor-type fuel injector systems having a single centralized high pressure pump and a distributor valve for metering and timing fuel flow from the pump to each of a plurality of injection nozzles, such as disclosed in Japanese Application No. 57-68532 (Komatsu), are less expensive to construct than are other types of injection systems. However, distribution-type systems are not as reliable in operation as other types of systems due to unpredictable/uncontrollable behavior of high pressure fluids within the fluid line connecting the centralized high pressure fuel pump to the individual injector nozzles.
Many of the drawbacks associated with distributor-type systems can be overcome by providing an individual cam operated unit injector at each engine cylinder location, such as illustrated in U.S. Pat. No. 4,392,612, whereby only low pressure fuel needs to be supplied to each injector, since the high pressure necessary for injection can be supplied by the cam actuated pump located in each injector immediately adjacent the engine cylinder. Each injector also includes a control valve, e.g. solenoid valve, mounted on the injector body to control the amount of fuel injected into each cylinder. However, the requirement of an individual pump and control valve for each injector creates substantially higher manufacturing costs as compared with distributor-type systems. In addition, the unit injectors disclosed in U.S. Pat. No. 4,392,612, are designed so that each solenoid valve must close and open during a single injection stroke of the injector pump or plunger as the plunger moves inwardly to control the beginning and end of injection, respectively. Since each injection stroke of the plunger must occur in an extremely short period of time near the top dead center position of the corresponding engine piston as it completes the compression stroke and commences the power stroke, the design, operation and control of the solenoid valve becomes a critical, and often costly, consideration in the design of the unit fuel injector. In fact, it has been found that these types of unit injectors are not always capable of achieving predictable and effective control of the timing and metering of fuel injection over a wide range of operating conditions.
Commercially competitive fuel injector systems of the future will almost certainly need some capacity for controlling the timing of injection completely independent from the quantity in response to changing engine conditions in order to achieve acceptable pollution abatement and fuel efficiency. Certainly, some emission control standards will be difficult or impossible to meet unless both timing and quantity of fuel can be controlled extremely accurately on a cycle-by-cycle basis depending on operator demand and engine conditions. However, achieving the high degree of control required in high pressure distributor-type systems will be extremely difficult due to the high pressure waves transmitted through the high pressure lines connecting the distributor pump with the individual injectors. Likewise, although numerous attempts have been made to design a unit injector system which provides for variable timing and metering, a unit fuel injector system which is both economical and highly accurate has not yet been achieved.
U.S. Pat. Nos. 4,281,792 and 4,531,672, provide examples of attempts to solve this dilemma by disclosing unit fuel injectors which attempt to achieve independent control over injection timing and metering while minimizing the demands on the solenoid valve. The unit injector disclosed in U.S. Pat. No. 4,281,792 includes a two-part plunger having a variable volume hydraulic chamber separating the plunger sections and a single solenoid valve which commences the injection on the inward stroke of the plunger by closing to form a hydraulic link between the plunger sections. The point of closure can be varied to vary the point at which injection commences as illustrated by points B,C and D of FIGS. 8 and 9 of the '792 patent. Because points B,C and D are located on a rotatively steep portion of the cam surface, the point of closure of the control valve is quite time sensitive. On the outward stroke, the solenoid valve opens at a selected point to control the quantity of fuel metered for injection on the subsequent downstroke. The point of opening is illustrated by point E which may occur over a relatively less steeply sloped portion of the curve and such opening is less time sensitive. Therefore, this design eliminates the need for the solenoid to control both timing and metering in the relatively short time period of the inward stroke of the plunger. However, since the solenoid must still operate during the inward stroke to control timing and the inward stroke must occur over a relatively short time period (steep portion of cam profile) within the total cycle time of the engine piston, operating requirements for the solenoid and its associated circuitry still remain high.
U.S. Pat. No. 4,531,672 further minimizes the operating requirements of the solenoid valve by providing a unit injector which operates only on the outward stroke of the plunger, or during a dwell when the plunger is not moving, to control both timing and metering. As a result, a greater period of time, or window of opportunity, is provided within which the solenoid may operate. However, this fuel system, like the one disclosed in U.S. Pat. No. 4,281,792, does not entirely separate the timing and metering functions of each unit injector primarily because a single solenoid and associated supply passage serves both the metering and timing passages for each injector. As a result, the metering and timing phases can not occur at the same time. Consequently, the window of opportunity for metering corresponding to a single outward stroke of the plunger must be allocated between the metering and timing phases thereby undesirably decreasing the amount of time available for the completion of each phase. Moreover, within a given window of opportunity for metering, the single solenoid valve must be accurately controlled to open and then close with respect to the opening and closing of a supply or drain port by the plunger.
Unit injector systems such as those disclosed in U.S. Pat. Nos. 4,281,792 and 4,392,612 also suffer from the disadvantages inherent in systems having individual solenoid valves associated with each unit injector. Unlike a more conventional open nozzle unit injector, for example as disclosed in FIG. 16 of U.S. Pat. No. 3,951,117, which operates on pressure/time principles to control both metering and timing and therefore does not require a solenoid valve, these solenoid operated unit injectors require a solenoid valve for each injector resulting in a more complex and costly injector. In addition, the injector barrel must be forged to include a boss for receiving the solenoid valve body instead of using the simpler screw machining process for producing a symmetric injector body. Also, the boss and solenoid assembly extend into the cylinder head adjacent the injector restricting the space available for other engine components, such as the injector and valve drive train assemblies, while increasing the overall size of the engine. Lastly, many of these solenoid valves must be designed to withstand the extremely high pressures of the timing or metering fluids under compression by the injector plunger thus increasing the cost of the injector.
As mentioned above, the open nozzle fuel injector, such as disclosed in FIG. 16 of U.S. Pat. No. 3,951,117 and in FIG. 1 of U.S. Pat. Nos. 4,971,016 and 5,042,445, avoids the need for a solenoid valve since the amount of injection fuel and timing fluid metered to the injector is controlled by pressure-time metering, that is, the pressure of the fuel or fluid supplied to the injector through a precisely dimensioned feed orifice and the time period the plunger uncovers the feed orifice. However, this type of pressure-time control requires the fuel pressure to be constantly and accurately varied in response to changing engine conditions. To achieve this goal, many of these systems include pressure transducers in the supply lines to each injector for sensing the fuel supply pressure and providing feedback to the pressure controller thus adding to the overall cost of the fuel system. Moreover, open nozzle pressure-time fuel injector systems do not allow for individual cylinder control since fuel and timing fluid is constantly fed to each injector through a pressure regulator. In order to improve emissions and fuel economy, it is occasionally desirable to prevent one or more selected cylinders from providing power to the engine by stopping the injection of fuel into the combustion chamber by the injector corresponding to the particular cylinder or cylinders. However, this type of cylinder "cut out" is not practical with open nozzle, pressure-time, common rail injectors since a single injector cannot be easily isolated from the other injectors during operation of the engine.
Another problem associated with open nozzle pressure-time injectors is the inability of the injector to provide fast, positive response to fuel supply pressure changes. The amount of fuel metered is controlled at least in part by the fuel supply rail pressure which is varied depending on various engine conditions. When the fuel supply pressure is sharply decreased in response to changing engine conditions, it takes a period of time for the fuel pressure in the supply passage adjacent the injector to decrease to the new pressure level. This delay in response impedes the ability of the pressure-time metering control system to provide fast, accurate control of timing and metering.
Another problem commonly experienced in open nozzle pressure-time injectors is the presence of combustion gases in the supply passage between the supply port and the inlet check valve. Gases from the combustion chamber are pushed up into the supply passage by the engine piston. These gases interfere with the control of fuel metering and, therefore, must be removed. One attempt to remove the gases includes forming a scavenging flow passage in the supply side distinct from the supply or feed port for directing the gas containing supply fuel through the injector to drain creating a scavenging effect. However, such efforts to remove the gases have not always been completely successful. Similarly, the combustion gas or cylinder pressure may affect the amount of fuel metered in another way. The supply fuel must be metered against the cylinder pressure acting up through the metering chamber of the injector even though no gas actually travels to the fuel supply. At the relatively low fuel pressures necessary at low operating speeds and loads for efficient operation of open nozzle pressure-time injectors, the effects of cylinder pressure on fuel metering can be substantial resulting in yet another variable which must be considered before achieving accurate control of metering throughout the range of operating conditions.
Other fuel injection systems have been developed in an attempt to overcome some of the deficiencies discussed above while also attempting to achieve efficiency of combustion, fuel economy and emissions abatement. In order to achieve these goals, it is certain that the fuel supply system must be able to provide precisely controlled amounts of fuel and timing fluid to each injector at the precise time required in the injection cycle. U.S. Pat. No. 4,621,605 provides an example of such an attempt by disclosing a positive displacement fuel injection system which forms and delivers pre-metered slugs of fuel and timing fluid to unit injectors. This system is capable of varying the size of the fuel and timing slugs on a cycle-by-cycle basis without the use of individual solenoid valves and pressure-time metering. However, the system uses a complex fuel pump including a piston/chamber arrangement, variable position mechanical stop and a 3-way flow control valve for each fuel metering and timing fluid circuit. Consequently, the system is complicated, costly and impractical for many purposes. Moreover, the slug forming chambers are remote from each cylinder which adds line condition variables that are not necessarily controllable or predictable. In addition, since only one fuel metering and one timing control arrangement serve all injectors of the engine, each control arrangement must deliver, in the time period of a given engine cycle, a number of metered slugs corresponding to the total number of injectors in the engine. Therefore, the total time period of a complete cycle of the engine must be allocated into a number windows of opportunity for metering corresponding to the total number of injectors in the engine, e.g. six windows for a six cylinder engine. As a result, the window of opportunity for metering for each injector cannot be maximized in the total engine cycle time period and the operating requirements, e.g. response time, of the control arrangement must be very high.
As previously discussed, U.S. Pat. No. 4,531,672 to Smith discloses a unit fuel injector containing a fluid timing circuit and a fluid metering circuit for providing fuel flow to respective timing and metering chambers by means of a single solenoid valve which is adapted to control separately timing and metering through variation in the time of opening and closing, respectively, during each cycle of operation. While this type of injector design may provide adequate control over both timing and metering, it uses common metering and timing passages thereby requiring engine fuel to be used as the timing fluid..As a result, a greater amount of fuel is supplied to the unit injector than is necessary to supply the injection chamber since fuel is continually cycled through the timing chamber during injector operation. This results in a substantial amount of timing fuel being heated within the injector and subsequently drained or spilled to the fuel supply tank. The hot fuel returned to the supply tank causes undesired fuel evaporation and often requires the installation of fuel cooling heat exchangers to reduce the temperature of the fuel in the supply tank.
The problems associated with draining excessive quantities of hot fuel to the supply tank and the accompanying pressure spikes have become even more apparent due to recent and upcoming legislation placing strict emission standards on engine manufacturers resulting from a concern to improve fuel economy and reduce emissions. In order for new engines to meet these standards, it is necessary to produce fuel injectors and systems capable of achieving higher injection pressures, shorter injection durations and more accurate control of injection timing. High injection pressures may be achieved in a number of ways such as by varying the cam profile, plunger diameter and/or number and size of injection orifices. Various techniques have been developed to control timing including mechanical, e.g. racks for rotating injector plungers having helical control surfaces; electronic, e.g. valves for controlling the start and/or end of injection and hydraulic, e.g. variable length hydraulic links. With respect to the latter, timing is advanced by introducing more timing fluid into the timing chamber which effectively lengthens the fluid link between the injector plungers. In the typical injector, as a result of this lengthened link, the pumping plunger commences injection and/or reaches its bottom most position at an earlier point in the rotation of the corresponding cam. Accordingly, fuel injection can occur at a point in the combustion cycle when the piston of the engine is still moving upward.
Because fuel is normally used as the timing fluid in injectors of this type, the amount of fuel which is supplied to and drained away from the injector of an engine necessarily increases as compared with injectors employing non-hydraulic timing control or no timing control. The amount of heat absorbed by the fuel and ultimately the temperature of the fuel in the fuel supply tank has been found to increase to an unacceptably high level.
Other fuel injector and fuel injection system designs which provide for variable timing and metering are disclosed in U.S. Pat. Nos. 4,249,499 to Perr and 4,410,138 to Peters et at. The unit injector design disclosed in the '499 Perr patent includes a timing mechanism having movable pistons connected between a cam drive and an injector plunger that allow timing fluid to enter a timing chamber to form a variable length hydraulic link between the pistons depending on the pressure of the supply wherein the length of the link determines the point at which injection is initiated. The timing fluid circuit, which preferably uses engine lubricant, is separate from the fuel supply or metering circuit. Therefore, since lube oil is used as a timing fluid in a separate timing circuit, the above-mentioned hot fuel drain problem is avoided in this design. However, this design controls injector timing using a variable pressure timing fluid mechanism, while fuel metering control is based on pressure-time metering. Consequently, both timing fluid pressure and metering fuel pressure are critical variables which must be carefully controlled for proper timing and metering. Precise control of fuel and fluid pressure to accurately and effectively control both fuel injection timing and metering over a wide range of operating conditions is often difficult to achieve.
U.S. Pat. No. 4,410,138 to Peters et al. discloses a fuel injector having infinitely variable timing using a two part injector plunger which forms a variable link timing chamber between the upper and lower plungers for receiving timing fluid. Here again, although the timing fluid circuit is completely separate from the fuel metering circuit, precise control of both the timing fluid pressure and metering fuel pressure are necessary for accurate and reliable control of timing and metering.
U.S. Pat. No. 5,143,291 to Grinsteiner discloses a unit fuel injector using high pressure lubricating oil to pressurize the fuel for injection. However, each fuel injector requires a separate solenoid valve for controlling the flow of lubricating oil resulting in a more complex and costly injector. Also, the lubricating oil enters each injection at high pressure and is not compressed in a timing chamber by an engine-operated timing plunger. Therefore, the lubricating oil in each injector does not experience temperature increases associated with the high compression of timing fluid in injectors having mechanically driven pump plungers.
Another important concern accentuated by higher injection pressures is the need to adequately cool unit injectors during operation. In the fuel injector design disclosed in U.S. Pat. No. 4,531,672 to Smith, both the metering fuel and the timing fuel inherently function to cool the unit injector. However, it has been discovered that when fuel is used as the timing fluid, excessive heat may be absorbed by the fuel resulting in the fuel assuming an unacceptably high temperature over extended periods of engine operation. Thus, in order to ensure adequate cooling of the injector, the fuel in the fuel supply tank must be cooled using expensive coolers.
Another important requirement of fuel injectors using engine fuel as timing fluid is to provide a leak off passage between the uppermost plunger and the rocker arm or driving assembly. Without such a leak off passage, fuel leakage by the uppermost plunger would cause the fuel to be mixed with the engine lubrication oil supplied to the rocker arm and linkage assembly impairing the lubrication qualities of the lube oil and ultimately increasing engine wear.
Consequently, there is a need for a simple, reliable, low cost yet high performance fuel injection system which can effectively and predictably control both fuel injection timing and metering by maximizing the time period available for metering of fuel and timing fluid. There is also a need for such a fuel injection system which can effectively and predictably control both fuel injection timing and metering while adequately cooling the injector internals without causing excessive heating of the engine fuel.